(1) Field of the Invention
This invention relates to a so-called "Run Flat Tire" which has a reinforcing rubber layer in the entire region of the sidewall. The layer has a crescent shaped cross section in order to enable vehicles to run over long distance when the tire is punctured. More particularly, this invention relates to a tire with good run flat durability which can be run over a long distance when punctured without developing trouble as compared with conventional run flat tires by reinforcing the tread portion of the tire properly.
(2) Related Art Statement
In order to enable a vehicle to continue to run safely for a relatively short distance and get to a location the driver desires, for example, a service station where the tire can be replaced there without disadvantages such as further damage to the tire or loss of maneuverbility when inner pressure of the tire fitted to the vehicle is reduced or completely deflated by some trouble mainly such as nails or metal pieces or the like (called "puncture" hereinafter), a lot of solutions have been provided.
Among them, employment of a reinforcing layer in the entire region of the side wall portions is a so called "Run Flat Tire" and is generally adopted as a most efficient measure. In this solution, a load of the tire is supported mainly by normal inner pressure and secondarily by the reinforcing layer under the normal operating conditions. The load is taken over by the reinforcing layer alone in the punctured state.
In the punctured state, the sidewall portions of a pneumatic tire, without a reinforcing layer, cannot stand the wheel weight, and are crushed into a conduplicate shape. This results in a fatal destruction of the tire and loss of dynamic properties. In the run flat tire, however, although the sidewall portions are bent considerably, the pneumatic radial tire can continue to run by adopting the reinforcing layer with shape and material capable of bearing wheel weight for a relatively short distance without damaging the component of the sidewall portions and still providing control of the vehicle.
The above mentioned run flat property is generally adopted to a high performance tire. In this regard, various dynamic properties such as steering property and anti vibration riding comfortability to be performed throughout the driving period must be maintained at a maximum level. Therefore, any deterioration of functional tire performance brought by adopting a run flat property used for just the puncture contingency must be minimumized.
In the selection of cross section shape and material of the reinforcing rubber layer, a decision and adoption within a limited thickness and hardness of the layer are necessary. Consequently, though it can be run safely for a certain distance, a limitation for running distance occurs. Though it has a structural disadvantage for run flat property, currently a radial carcass pneumatic tire has become popular. To satisfy the demand for extending safety running distance in the puncture state, prior run flat tires no longer meet this requirement, and improvement has been desired.